Originally Posted by
cwatters
Most system use negative earth these days. Can't remember last time I saw a positive earth system. Probably was in a 1970's car?
A significant user is the London Underground.
Most electric railways use negative earth, with a straightforward power supply through an overhead wire or third rail, returning through the regular rails and the ground. However the London Underground, uniquely, uses two separate power rails, a positive which is outside the track and a negative which is in the centre. You can look for this the next time you are on the Tube. Furthermore the voltage is 630v DC. This is not all in the positive rail; it is +420v in the positive outside rail, and -210v in the negative middle rail, net potential difference is 630v. There are reasons for this fully insulated system, to do with the iron lining of tube tunnels, and the avoidance of corrosion by electrolysis due to ground currents.
Notably some lines of the Underground continue on the National rail system out in the open, who do not do it this way. They still provide the middle rail specially for the Underground trains, but it is at 0 earth potential, and just bonded to the ground, while the outside power rail is at +630v DC. The Underground train sees the potential difference exactly the same. Trains run across the transition between the two at the boundary without any issue. The National railway trains which share these tracks just run with their normal +630v in the outside rail, the current returning through the wheels and tracks/ground as elsewhere on the system not shared with the Underground.